I don't know for sure, but I've done some research since I will be doing this soon myself.
The H23 has about 160-170hp stock.
The H22 Vtec has about 200hp stock.
The H23 has a longer stroke, but the cylinders are the same as the H22 which is why you can bolt on the H22 head.
The H22 has a higher compression ratio because of the pistons, and the combustion chamber.
If you put the H22 pistons in the H23 with the H22 head you should get a higher compression than either, and more HP, but you will have to run high test.
The theoretical figures I got off another website were somewhere around 11:1 compression ratio.
In addition, you will need a computer that will engage the Vtec.
You can get one from a junkyard, or online.
I bought an AEM Plug & Play computer for my H23, but when I swap the H22 Vtec in next week I can still use it because it has a built in Vtec controller.
You can also use it to controll boost, and just about anything else you can think of adding.
More engine parts needed:
All the sprockets that mesh with the cogs on the timing belt.
The H22 timing belt is different.
H22 timing belt.
Might as well replace the tensioner, and water pump while you're at it.
Intake header
Exhaust header
H22 head gasket/set
The oil plug in the H23 block can indeed be removed easily. A small screwdriver will work, but it is threaded on the top, and one of the bolts you take off the motor while disasembling it will work as well.
One person suggested that there was some kind of "oil slinger" in the H22 block which they recommended switching over as well, but didn't think it was mandatory.
Thats pretty much all I can think of. If anyone can disprove any of this, or has confirmation, please let me know. Like I said... I'm not sure... Just going off what I've read in other blogs.